2-Keto-L-gulonic acid is an important intermediate for the production of L-ascorbic acid, into which it can be converted.
The production of 2-keto-L-gulonic acid from L-sorbose or from D-sorbitol by fermentation is known.
As to L-sorbose, as disclosed in "Acta Microbiologica Sinica" 21(2), 185-191 (1981), 2-keto-L-gulonic acid can be produced from L-sorbose by a mixed culture of microorganisms, specifically Pseudomonas striata and Gluconobacter oxydans. The yield of this process is 30 g/l from a starting concentration of 70 g/l of L-sorbose. and 37 g/l from a starting concentration of 100 g/l of L-sorbose. European Patent Publication No. 0221 707 discloses the production of 2-keto-L-gulonic acid from L-sorbose by Pseudogluconobacter saccharoketogenes with and without concomitant bacteria. However, the yield of this process using Pseudogluconobacter saccharoketogenes is at most 55.3-87.6 g/l (conversion ratio: 34.2-54.1%)(See:page 13, Table 4 of European Patent Publication No. 0221 707). European Patent Publication No. 0278 447 discloses a process for the production of 2-keto-L-gulonic acid from L-sorbose by a mixed culture of microorganisms, one of which has the identifying characteristics of strain DSM No. 4025 and the other of which has the identifying characteristics of DSM No. 4026 (a Bacillus megaterium strain). The yield of this process is at least 40 g/l.
As to D-sorbitol, Japanese Patent Publication No. 40154/1976 discloses the production of 2-keto-L-gulonic acid from D-sorbitol by microorganisms of the genus Acetobacter, Bacterium or Pseudomonas. These microorganisms are capable of oxidizing D-sorbitol under aerobic condition producing 2-keto-L-gulonic acid. This process gives a low yield of less than 6 g/l. D-sorbitol as a starting material would be preferable to L-sorbose because it is less costly. However, the yield of 2-keto-L-gulonic acid from D-sorbitol is low.
The above processes represent attempts to microbiologically produce 2-keto-L-gulonic acid directly from either L-sorbose or D-sorbitol as starting materials. However due to the relatively low yields these processes are far from an industrial scale of production, especially when D-sorbitol is the starting material.
On the other hand, the fermentative production of L-sorbose from D-sorbitol is known. Various Acetobacter (presently classified into Gluconobacter) strains such as Acetobacter xylinum and Acetobacter suboxydans are known to produce L-sorbose from D-sorbitol efficiently (Biotechnology, volume 6a, 436-437, 1984, edited by H.-J. Rehm and G. Reed, published by Verlag Chemie, Weinheim, Germany). D-Sorbitol is a less costly material than L-sorbose, but L-sorbose is more efficiently converted to 2-keto-L-gulonic acid. Therefore, a process that uses D-sorbitol as a starting material, but provides L-sorbose for conversion, combines the availability of D-sorbitol with the higher yield provided by L-sorbose. If part of the process is the conversion of D-sorbitol to L-sorbose, which is then converted to 2-keto-L-gulonic acid, then efficient production will be attained.
Provided that one can establish an efficient production process of 2-keto-L-gulonic acid starting from a cheaper carbon source such as D-sorbitol rather than from L-sorbose, it is obvious that production will be dramatically simplified.